Capillary evaporators are used in a variety of two-phase thermal management systems. The primary difference between capillary evaporators and flow-through and kettle boilers is that nucleate boiling does not occur in evaporators, whereas it does in boilers. Instead, evaporation takes place in a capillary evaporator at a liquid-vapor interface held stable by a capillary wick structure. The liquid supplied to an evaporator is at a pressure lower than the vapor pressure, and the liquid is drawn into the evaporator by the capillary suction of the wick.
A common capillary evaporator configuration is the configuration used in heat pipes. A conventional heat pipe typically consists of a tube containing a porous capillary wick layer in contact with the inner surface of the tube. One portion of the heat pipe, typically one end, absorbs heat from a heat source and functions as an evaporator. Another portion, typically the other end, rejects heat to a heat sink and functions as a condenser. The capillary wick returns the liquid from the condenser portion to the evaporator portion of the heat pipe via the capillary pumping action of the wick. The inner surface of the wick defines a central passageway that conducts vapor from the evaporator portion to the condenser portion of the heat pipe. The capillary wick can be any of a variety of structures, such as machined grooves, a discrete metal screen, sintered metal powder, or a plasma-deposited porous coating. Heat pipes are economical to fabricate and work well in applications with modest heat fluxes and relatively short heat transport distances. Many contemporary high-performance laptop computers use heat pipes to remove heat from the processor and transfer it to the case.
Within a heat pipe, the liquid has to flow a substantial distance from the condenser portion to the evaporator portion through the capillary wick. This creates a large pressure drop for the liquid that effectively limits the maximum liquid flow rate, thereby limiting the heat transport capacity of the heat pipe. If the pore size of the wick is decreased to provide higher capillary suction, the permeability of the wick decreases and the pressure drop increases. Increasing the thickness of the wick reduces the pressure drop, but increases the distance the heat must be conducted through the wick at the evaporator portion of the heat pipe. Increasing the thickness of the wick translates into a higher thermal resistance at the evaporator and, perhaps more limiting, an increase in the liquid superheat at the interface between the inner surface of the tube and the wick. Eventually, the superheat at the base of the wick becomes too large and boiling takes place in the wick, leading to a drying out of the wick. When the wick dries out, the performance of the wick degrades substantially.
Many applications, including spacecraft thermal management systems, need higher heat transport capacity over longer distances than afforded by conventional heat pipes. For these applications, the basic heat pipe is typically enhanced by returning the liquid from the condenser portion to the evaporator portion in a separate tube that does not have an internal wick. Because this return flow does not suffer the large pressure drop of flow through a wick, the distance between the evaporator and condenser can be substantially increased. Also, the capillary wick within the evaporator is moved away from the heat-acquisition interface, typically by providing ribs that additionally define vapor passageways between the wick and heat-acquisition interface. These modifications lead to two types of heat-transfer systems, namely, the loop heat pipe (LHP) and capillary pumped loop (CPL). CPLs and LHPs are increasingly being employed in spacecraft thermal management systems, and their operating characteristics, both on earth and in microgravity, have been studied extensively.
FIG. 1A shows an exemplary conventional evaporator suitable for use in either an LHP or CPL. Evaporator 20 includes a tubular housing 22 and a like-shaped capillary wick 24 located within the housing. Capillary wick 24 defines a central passageway 26 for conducting a liquid 28 along the length of the wick. Housing 22 is typically made of a highly conductive metal and includes a plurality of ribs 30. Ribs 30 serve the dual purposes of: (1) defining a plurality of vapor passageways, or channels 32, for conducting vapor 34 formed by vaporizing liquid 28 away from capillary wick 24 and (2) conducting heat from the outer portion of housing 22 to the capillary wick to transfer the heat to the liquid, thereby causing the liquid to vaporize.
The primary differences between conventional evaporators of CPLs and LHPs, such as evaporator 20, and the evaporator portions of conventional heat pipes are that in the LHP/CPL type evaporators the liquid supply is substantially thermally isolated from the heat source, e.g., by capillary wick 24, and the liquid flow through the capillary wick is normal to the heat acquisition interface and, hence, the flow area is much larger and the flow length much shorter than in the “wall-wick” evaporator portion of a heat pipe. These differences result in substantially higher heat transport capacity for LHPs and CPLs than for heat pipes. However, the higher heat transport capacity in LHP/CPL type evaporators comes at a price, namely, a substantially degraded thermal connection between heat source 36 and capillary wick 24 caused by the non-continuous contact of housing 22 with the wick via ribs 30, which are typically made of metal.
The design of metal ribs 30 must meet the conflicting requirements of minimizing the thermal resistance between housing 22 and capillary wick 24, while at the same time minimizing the vapor pressure drop within evaporator 20. As shown in FIG. 1B, the presence of ribs 30 distorts the heat transfer and fluid flow in capillary wick 24 because they create hot zones within the wick. At low heat fluxes, capillary wick 24 is completely wetted and evaporation takes place only in regions 33 immediately surrounding the edges of the ribs 30 where the ribs contact the wick. The magnitude of heat transfer is limited by the perimeter length of the ribs that contact the wick. The total area of evaporation regions 33 in capillary wick 24 is therefore small and, hence, the evaporation resistance much increased. Additionally, instead of flowing uniformly through capillary wick 24, liquid 28 must now converge into narrow regions along ribs 30, greatly increasing the pressure drop in the wick.
FIG. 1C shows conditions that exist within the wick at large values of heat flux. At higher heat fluxes, the liquid-vapor interface 40 recedes into capillary wick 24, providing a larger area for evaporation. As liquid-vapor interface 40 recedes, the thermal resistance of evaporator 20 increases because of the relatively low thermal conductivity of capillary wick 24. Perhaps more importantly, as liquid-vapor interface 40 recedes, the overall pressure drop increases sharply because vapor 34 must now flow some distance through the small pores of capillary wick 24 before reaching vapor channels 32. Eventually, the pressure drop in vapor 34 exceeds the capillary pumping capacity of capillary wick 24 and the vapor breaks through to central passageway 26, i.e., the liquid side of evaporator 20. This “vapor blow-by” condition sets a heat flux limit on evaporator performance.
To mitigate these effects, conventional LHP-type evaporators typically have metal capillary wicks instead of ceramic, glass, or polymer wicks to provide the wicks with a relatively high thermal conductivity. Higher thermal conductivity more effectively spreads heat into the wick, increasing the area over which evaporation takes place, thereby reducing thermal resistance. However, higher thermally conductive wicks increase the leakage of heat through the wick to liquid 28 at the other side of the wick. This can cause boiling of liquid 28 in the central passageway 26 thereby blocking the flow of liquid 28 to the evaporator and limiting the maximum heat flux. Increasing the thickness of the wicks will somewhat mitigate this heat leakage but will, in turn, decrease their permeability and, thus, also reduce the maximum heat flux of such evaporators.
It is anticipated that thermal management of future high-power laser instrumentation, next- and future-generation microprocessor chips, and other electronics, among other devices, will require power dissipation in the range of 2-5 kW at heat fluxes greater than 100 W/cm2 The ITANIUM® microprocessor from Intel Corporation, Santa Clara, Calif. is already reaching local heat fluxes of about 300 W/cm2. In contrast, most conventional evaporators, such as evaporator 20 discussed above, typically do not work at heat-fluxes in excess of about 12 W/cm2 because vapor blanketing in the capillary wicks blocks the flow of liquid into the wicks. Although some more recent evaporator designs, such as the bidispersed wick design, have demonstrated good performance at localized heat fluxes of 100 W/cm2, there is, and will continue to be, a need for evaporators capable of routinely handling average heat fluxes of 100 W/cm2 and greater.